Measuring anatomical features might be expected to be done on a three-dimensional data set, such as can be reconstructed from data taken during a computed tomography (CT) scan or a magnetic resonance image (MRI) scan. However, equipment for CT and MRI scans are expensive and bulky. Furthermore, they may not be available when and where needed. For example, CT and MRI equipment is usually not available for use during a medical intervention procedure, or within an operating room or treatment area.
Where knowledge of the anatomical feature is very important, a CT or MRI scan might be taken in advance of the procedure and the procedure planned based on it. However, there are many more medical procedures and interventions that might benefit from knowledge of the size of an anatomical feature in the patient, where the cost a CT scan is not justified or CT equipment is not available. For example, to treat a tumor or aneurysm, it is often helpful to know the size of the tumor or occlusion. Similarly, it would be helpful to know in advance of surgical interventions that involve implanting stents, screws, nails, or other devices in the body how well a chosen device will fit, or if a device of a different size or shape would be more appropriate.